Optical cable methods of manufacture thereof and articles comprising the same

ABSTRACT

where d is the amount of optical fiber clearance for free movement within the flexible protective tube, D is an average helical diameter of the helically wound flexible protective tubes, and p is an average helical pitch of the helically wound flexible protective tubes.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 16/040,950, filed Jul. 20, 2018, by Dyer, et al., which claimsthe benefit of U.S. provisional patent application Ser. No. 62/536,575,filed Jul. 25, 2017, by Dyer et al., all of which are incorporatedherein by reference in their entireties.

BACKGROUND

This disclosure relates to an optical cable used for sensing, methods ofmanufacture thereof and to an article comprising the same. Morespecifically, this disclosure relates to an optical cable used forsensing temperature and strain, methods of manufacture thereof and to anarticle comprising the same.

In applications such as offshore oil pipelines, in order to aid in theflow of oil through the pipeline, the pipe is heated and the pipelinetemperature controlled. When the pipe with the sensing cable installedis loaded onto a boat, transported and offloaded for deployment, duringthe steps of the installation process, the pipe can be exposed to bothtensile and compressive forces resulting in a positive (+) or negative(−) change in length (strain). The optical fiber cable sensor attachedto the pipe must be able to withstand these changes in length withouttransmitting any detrimental stress onto the optical fiber. Excessivestress on the optical cable due to this mechanical deformation for anextended period of time could result in premature mechanical failure ofthe optical fiber (which is part of the optical cable). If any of thestress on the optical cable is transferred and permanently imparted tothe optical fiber, the accuracy of the temperature measurement inapplication is adversely affected. It is therefore desirable in cabledesign to reduce the strain that the optical fibers experience duringinstallation to avoid any detrimental effect to the reliability of theoptical cable.

In addition, in offshore applications the pipeline may see temperaturesfluctuations as low as −30° C. and up to +120° C. These temperaturechanges will cause the fibers to expand (+) and contract (−). It isdesirable to permit the fibers to expand or contract while isolating thesensing optical fibers from mechanical or environmental strain orcompression in order to ensure both accuracy in temperature measurementsand to assure long-term survival following manufacture and installation.It is therefore also desirable for cable design to accommodate stressesinduced by the deformation caused by temperature change.

SUMMARY

Disclosed herein is an optical cable comprising a plurality of cablesensors helically wound around a support; and an outer jacket that isdisposed on the plurality of cable sensors and surrounds the pluralityof cable sensors; where each cable sensor comprises an optical fiber;where the optical fiber comprises an optical core upon which is disposeda cladding; a primary coating; a deformable material surrounding theoptical fiber; and an outer tube surrounding the deformable material;where the optical fiber is longer than the outer tube by an amount of0.1 to 2%; and where the strain in the cable sensor is determined byequations (1) and (2) below:

$\begin{matrix}{ɛ = {{\frac{\sqrt{{\pi^{2}\left( {D + \frac{d}{2}} \right)}^{2} + p^{2}}}{p} - \frac{\sqrt{{\pi^{2}\left( {D - \frac{d}{2}} \right)}^{2} + p^{2}}}{p}} = {\frac{\pi^{2}{dD}}{p^{2}} = \frac{10dD}{p^{2}}}}} & (1) \\{{ɛ \times 100} = {{Percent}\mspace{14mu}{elongation}\mspace{14mu}{or}\mspace{14mu}{contraction}}} & (2)\end{matrix}$where d is the amount of fiber clearance for free movement of thesensing fiber within the loose tube, D is an average pitch diameter ofthe plurality of cable sensors and p is an average pitch length of ahelical turn of the plurality of cable sensors wound around the centralstrength member.

Disclosed herein is a method of manufacturing an optical cablecomprising helically winding a plurality of cable sensors around asupport; and disposing an outer jacket on the plurality of cable sensorssuch that the outer jacket surrounds the plurality of cable sensors;where each cable sensor comprises an optical fiber; where the opticalfiber comprises an optical core upon which is disposed a cladding; aprimary coating; a deformable material surrounding the optical fiber;and an outer tube surrounding the deformable material; where the opticalfiber is longer than the outer tube by an amount of 0.1 to 2%; and wherethe strain in the cable sensor is determined by equations (1) and (2)below:

$\begin{matrix}{ɛ = {{\frac{\sqrt{{\pi^{2}\left( {D + \frac{d}{2}} \right)}^{2} + p^{2}}}{p} - \frac{\sqrt{{\pi^{2}\left( {D - \frac{d}{2}} \right)}^{2} + p^{2}}}{p}} = {\frac{\pi^{2}{dD}}{p^{2}} = \frac{10dD}{p^{2}}}}} & (1) \\{{ɛ \times 100} = {{Percent}\mspace{14mu}{elongation}\mspace{14mu}{or}\mspace{14mu}{contraction}}} & (2)\end{matrix}$where d is the amount of fiber clearance for free movement of thesensing fiber within the loose tube, D is an average pitch diameter ofthe plurality of cable sensors and p is an average pitch length of ahelical turn of the plurality of cable sensors wound around the centralstrength member.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic depiction of one exemplary embodiment of theoptical cable;

FIG. 2 is a schematic depiction of an exemplary optical fiber that isdisposed in the optical cable;

FIG. 3 depicts one pitch of the helix of the cable sensor and how thedesign permits the optical fibers to move freely in the radial directionwhen the optical cable is subjected to contraction, to elongation or isin the normal position;

FIG. 4 is a graph that depicts strain in the optical fiber for severaldifferent positions of the optical fiber within the cable sensor; and

FIG. 5 depicts a graph for a strain test conducted on the disclosedoptical cable where the x-axis is the sensing cable length and y-axis isthe Brillouin frequency.

DETAILED DESCRIPTION

Disclosed herein is a multi-fiber stranded loose tube cable (hereinafteroptical cable) with at least one optical fiber sensor per tube. Theoptical fiber sensors are hereinafter labelled “cable sensors”. In anexemplary embodiment, the optical cable comprises a plurality of loosetube buffered optical fiber sensors that are wound around a centralstrength member and bound together with a tape wrap and outer jacket.The overall optical cable diameter is less than 5 millimeters. Toachieve such a small cable optical diameter with the permitted allowablefree fiber movement (to accommodate for the temperature and stressvariations detailed above), an extremely short pitch length (p) is usedfor the cable sensors.

FIG. 1 is a schematic depiction of one exemplary embodiment of theoptical cable 300. The optical cable 300 comprises a plurality of loosetube buffered optical fibers (cable sensors—100A, 100B, 100C and 100D)that are wound around a central strength member 302 and bound togetherwith a tape wrap 304 and an outer jacket 306. The optical cable maycomprise 2 or more, preferably 3 or more, and more preferably 4 or morecable sensors. In a preferred embodiment, the optical cable 300comprises 4 cable sensors 100A, 100B, 100C and 100D.

The cable sensors 100A, 100B, 100C and 100D will now be detailed usingfiber 100A as an example. The cable sensor 100A comprises an opticalfiber 102 disposed in a flexible protective tube 104 that serves toprotect the fiber from temperature variations, variations in strain, orfrom other perturbations. The flexible tube 104 surrounds the opticalfiber 102 and contains a deformable material 106 that permitsdeformation (of the tube) without deformation of the optical fiber 102when the cable sensor 100A (or 100B, 100C and/or 100D) is subjected toany form of perturbation. The deformable material 106 is disposedbetween the optical fiber 102 and the flexible tube 104.

With reference to the FIG. 2, the optical fiber 102 (which serves as thesensor) comprises a core 202, a cladding 204 disposed on the core 202, acarbon layer 206 disposed on the cladding 204, a primary coating 208disposed on the carbon layer 206 and a secondary coating 210 disposed onthe primary coating 208. The optical fiber 102 within the flexible tube104 may be a single mode or multimode fiber. In an embodiment, the coreof the optical fiber 102 is a silica core 202 having an outer diameterof 5 to 100 micrometers, preferably 10 to 50 micrometers, and preferably15 to 35 micrometers. The core 202 may be doped with germania (GeO₂),phosphorus pentoxide (P₂O₅), alumina (Al₂O₃), titania (TiO₂), or acombination thereof, to raise the refractive index of the core.

The core 202 has a layer of cladding 204 concentrically applied over thefiber core. This cladding 204 has a lower refractive index than the core202 to confine the distributed acoustic signal traveling back along theoptical core. The cladding has an outer diameter in the range of 80 to250 micrometers.

The cladding 204 is made from silica with no doping, or alternatively,made from silica with a dopant that reduces the refractive indexrelative to the refractive index of the core. Suitable dopants that areused to reduce the refractive index of the cladding relative to the coreinclude fluorine, boron oxide, or a combination thereof.

The cladding 204 may optionally be coated with a layer of amorphouscarbon 206 to create a hermetic protective layer over the sensing fiber.The layer of amorphous carbon 206 is a hermetic coating that isimpervious to molecular water or hydrogen that may be present in thesensing environment. It has been shown that the ingress of hydrogen andwater into the silica glass core can cause crack growth from flaws thatmay already exist within the silica glass. Such crack growth couldresult in premature failure of the optical fiber sensor.

Coatings 208 (primary coating) and 210 (secondary coating) are appliedto the cladding or to the carbon layer during the production of theoptical fiber 102 to maintain the pristine condition of the core 202(e.g., the silica glass sensing fiber which exists within themanufacturing optical fiber drawing process). The coatings also providefor easier handing of the optical fiber sensor. The coatings arepreferably selected to meet the lower and upper service temperaturesencountered by the optical fiber as well as to protect the core of thefiber from harsh chemical environments that can be encountered in oilwells or in other mining operations.

It is desirable for the coatings to protect the optical fiber fromtemperatures of −50° C. to +150° C. These coatings are thermosetting andmay be cured with temperature (i.e. thermally cured) or with ultraviolet(UV) light. The coating may be applied in two or more layers. Theprimary coating 208 (also sometimes referred to a first layer) comprisesa very soft material. It may be a silicone or urethane based coating.This soft primary layer provides resistance to micro-bending within theoptical fiber when strained. Excessive micro-bending can result insignal loss within the optical fiber. The outside diameter of theprimary coating may be 120 to 250 micrometers.

A secondary coating 210 is disposed over the primary coating to providea harder protective shell to the coated optical fiber. The secondarycoating has a higher hardness than the primary coating. The hardersecondary coating also provides ease of handing of the optical sensingfiber. This harder layer may be a material such as cross-linkedacrylate, cured with temperature or ultraviolet light. In some instancesan extruded thermoplastic material may be used as the secondary coatingfor the optical acoustic sensing fiber. Examples of commerciallyavailable materials that are used to meet the environmental requirementsof the sensing application are polyvinylidene fluoride (KYNAR®),polytetrafluoroethylene (TEFLON®), polyurethanes, or a combinationthereof. The outside diameter of the secondary coating may be 170 to 320micrometers.

The optical fiber 102 has a length that is 0.1 to 2% longer than thelength of the flexible tube 104 that surrounds it. In an alternativeembodiment, the optical fiber 102 has a length that is 0.5 to 1.5%longer than the length of the flexible tube 104 that surrounds it. Inyet an alternative embodiment, the optical fiber 102 has a length thatis 0.7 to 1.4% longer than the length of the flexible tube 104 thatsurrounds it. This additional length permits the optical fiber to flex,bend, or to stretch, without any undesirable perturbation to the opticalor acoustic signals being transmitted along the core of the fiber whenthe flexible tube 104 is flexed, bent, or stretched in application.

With reference now again to the FIG. 1, the optical fiber 102 (which isused for sensing) is surrounded by a deformable material 106. Thedeformable material 106 permits the fiber to deform without imposing anystress or strain on the fiber that can cause signal deterioration. Thedeformable material is therefore a material that is easily compressed orthat has a low modulus of elasticity. The deformable material 106preferably does not interact with the primary or secondary coating.

In an embodiment, the deformable material 106 comprises a fluid such asair, inert gases such as nitrogen, argon, carbon dioxide; water, oil,organic liquids, or a combination thereof. A preferred fluid is air.

In another embodiment, the deformable material 106 can comprises anelastomeric material. Elastomeric materials are those that have anelastic modulus of less than 10⁶ pascals when measured at roomtemperature. Examples of elastomeric materials are polysiloxanes,polyurethanes, styrene-butadiene rubbers, polybutadiene, polyisoprene,styrene-butadiene-acrylonitrile rubbers, polychloroprene, perfluoroelastomers, fluorosilicone elastomers, fluoro elastomers, ethylene vinylacetate, polyetherimides, or the like, or a combination thereof.

The elastomers may be used in the form of a non-flowable solid (having aporosity of less than 10 volume percent), a foam (having a porosity ofgreater than 70 volume percent), a deformable flowable gel (that canflow without the application of any applied force other than gravity),or a combination thereof.

The deformable material 106 permits the optical fiber 102 to bend or todeform. The deformable material 106 undergoes deformation as a result offorces transmitted to it by the outer tube 104 and/or by the fiber 102.In undergoing deformation, the deformable material prevents damage fromoccurring to the optical fiber 102.

The outer tube 104 encloses the deformable material 106 and the opticalfiber 102. It is desirable for the material used to manufacture theouter tube to be highly flexible for use in long term dynamicapplications, have a low coefficient of thermal expansion andcontraction for dimensional stability over the operating temperaturerange, be capable of withstanding both a low and high servicetemperature, have a high tensile modulus for strength, display chemicalresistance for use in harsh environments, and display abrasionresistance for toughness and imperviousness to UV degradation forexposure to sunlight.

The outer tube 104 tube has an outer diameter of 1.5 to 2.0 millimeters(mm), preferably 1.6 to 1.9 mm and an inside (inner) diameter of 0.8 to1.2 mm, preferably 0.9 to 1.1 mm. These dimensions, outer and innerdimeter, may vary depending on the system installation factors, so longas an appropriate amount of EFL is contained within the tube. Thisconstruction also allows for the transmission of acoustic signalsthrough the tube material as well as through the deformable materialwithin the tube to the optical fiber sensor.

The outer tube 104 material preferably comprises a polymeric materialhaving a glass transition temperature of greater than 150° C. In anembodiment, the outer tube comprises a fluoropolymer, aperfluoropolymer, or copolymers thereof. Copolymers of the fluoropolymerare preferred. Suitable commercially available fluoropolymers for use inthe outer tube are KYNAR for polyvinylidene fluoride; TEFZEL forethylene tetrafluoroethylene; TEFLON, FLUON, DYNEON and NEOFLON.

With reference once again now to the FIG. 1, the optical cable 300comprises a plurality of loose tube buffered optical fibers (cablesensors—100A, 100B, 100C and 100D) that are wound around a centralstrength member 302 and bound together with a tape wrap 304 and an outerjacket 306. In an embodiment, the cable sensors 100A, 100B, 100C and100D are helically wound around the central strength member 302. Thisarrangement facilitates the accommodation of persistent strains on thecable sensors. Persistent strain at levels of 1%, on an optical fibermay result in premature mechanical failure. To avoid direct transfer ofexternal cable strain onto the optical fibers (the cable sensors 100A,100B, 100C and 100D) within the cable structure, the optical fibers areallowed to move freely both radially and longitudinally. In a strandedloose tube cable, the amount of allowable cable elongation and/orcontraction (ε) before fiber movement is restricted, is determined bythe amount of clearance for the optical fiber to move within the loosetube (d), the pitch diameter (D) and the pitch length (p) as shown inthe equations (1) and (2) below.

$\begin{matrix}{ɛ = {{\frac{\sqrt{{\pi^{2}\left( {D + \frac{d}{2}} \right)}^{2} + p^{2}}}{p} - \frac{\sqrt{{\pi^{2}\left( {D - \frac{d}{2}} \right)}^{2} + p^{2}}}{p}} = {\frac{\pi^{2}{dD}}{p^{2}} = \frac{10dD}{p^{2}}}}} & (1) \\{{{ɛ \times 100} = {{Percent}\mspace{14mu}{elongation}\mspace{14mu}{or}\mspace{14mu}{contraction}}},} & (2)\end{matrix}$

where d is the amount of clearance for the optical fiber to move withinthe loose outer tube, D is the average pitch diameter of the cablesensors 100A, 100B, 100C and 100D and p is the average pitch length ofthe helix of the cable sensors 100A, 100B, 100C or 100D wound around thecentral strength member 302 (the support 302) (See FIG. 1.). The amount“d” is calculated as the linear difference between the outer diameter ofthe optical fiber 102 and the inner diameter of the loose outer tube104. The outer diameter of the optical fiber 102 is subtracted from theinner diameter of the loose outer tube 104 to obtain the value of “d”.

FIG. 3 depicts one pitch of the helix of the cable sensor 100A and howthe design permits the optical fibers 102 to move freely in the radialdirection when the optical cable 300 is subjected to contraction, toelongation or is in the normal position. As detailed above, the opticalfiber 102 has a length that is of equal length to the flexible tube 104that surrounds it. The fiber is of the same length as the tube i.e. in aneutral position. The uniqueness of this design is the cable cancontract or elongate up to +/−2% while allowing stress free movement ofthe optical sensing fiber.

In addition, the inside diameter (ID) of the flexible tube 104 isseveral times (200 to 450%) larger than the diameter of the opticalfiber 102. As a result, the optical fiber 102 is free to move within thecable sensor 100A when the optical cable 300 is subjected to strain suchas when it is bent.

When the optical cable 300 is subjected to contraction (i.e., the pitchp is reduced to below an optimal length), the optical fiber 102 which isfree to move within the outer tube 104 (See FIG. 3) moves to the top ofthe outer tube 104. When the optical cable 300 is subjected to neithercontraction nor expansion (i.e., the pitch p is at an optimal length),the optical fiber 102 which is free to move within the loose outer tube104 (See FIG. 1.) stays at the center (i.e., a neutral position) of theouter tube 104. When the optical cable 300 is subjected to expansion(i.e., the pitch p is increased above an optimal length), the opticalfiber 102 which is free to move within the loose outer tube 104 (SeeFIG. 3.) moves to the bottom of the outer tube 104.

The average pitch length p and pitch diameter D are selected to maintainthe extended sections of the optical fiber 102 in a state close to zerostress and strain as much as possible. In an embodiment, the averagepitch p of the helically wound cable sensors 100A, 100B, 100C and 100Dis 10 to 100 millimeters, preferably 20 to 50 millimeters. In anotherembodiment, the average pitch diameter D of the cable sensor 100A, 100B,100C and 100D is 1 to 10 millimeters, preferably 2 to 4 millimeters. Itis desirable to choose values for the average pitch p and the pitchdiameter D that permit the optical fiber 102 to be located in the centerof the cable sensor 100A (during operation) so that is does not sufferundue stress.

The FIG. 4 contains a graph and several positions of the optical fiber102 within the cable sensor 100A and depicts how maintaining the opticalfiber 102 within the center of the cable sensor 100A produces minimalstress within the optical fiber 102. The graph is a plot of the strain(ε) in the optical fiber versus the strain (ε_(c)) in the cable. As maybe seen, by selecting optimal values for the pitch length p, pitchdiameter D, and allowable optical fiber movement within the loose tubed, a stress free region (2η) can be created and maximized in the cablesensors 100A, 100B, 100C and 100D, which facilitates accuratetemperature sensing using a Brillouin scatting based method.

With reference now again to the FIG. 1, the central strength member 302(hereinafter support 302) provides strength and support to the opticalcable 300. The support 302 is preferably a metal or polymeric wire thatis ductile and that can withstand temperatures to which the cable sensorwill be subjected to. In an embodiment, the central strength member 302comprises copper, aluminum, steel (e.g., carbon steel, stainless steel,or the like), titanium, bronze, brass, or a combination thereof.Suitable polymers are those that have melting points greater than 200°C. Examples of such polymers are polyetherimides, polyimides,polyethersulfones, polysulfones, polytetrafluoroethylene, epoxies,polyesters, or the like, or a combination thereof. In an exemplaryembodiment, the support 302 comprises a continuous fiber reinforcedpolymeric composite. An exemplary fiber reinforced polymeric compositeis a glass fiber reinforced epoxy composite.

The outer diameter of the support 302 is selected to permit the cablesensors 100A, 100B, 100C and 100D to be helically wound and to have apitch length p and diameter D that minimizes strain on the optical fiber102 and facilitates retention of the optical fiber 102 in the center ofthe respective cable sensors 100A, 100B, 100C and 100D. The centralstrength member 302 may be cylindrical or hollow (i.e., is tubular) andhas an outer diameter of 0.5 to 2.5 millimeters.

The tape wrap 304 is optional and comprises a tape that is operative toretain the cable sensors 100A, 100B, 100C and 100D in position on thesupport 302. The tape wrap 304 comprises a polymer that can withstandthe temperatures that the optical cable 300 will be subjected to withoutdegrading or reacting with other components in the optical cable 300.The tape is flexible (i.e., it can be mechanically deformed easily underaverage human force). The polymer used in the tape wrap 304 ispreferably a fluoropolymer, polyimide or polyetherimide that has amelting point above 150° C., preferably greater than 200° C. In anembodiment, the tape wrap 304 comprises a fluoropolymer, aperfluoropolymer, or copolymers thereof. Copolymers of the fluoropolymerare preferred. Suitable commercially available fluoropolymers for use inthe tape wrap are polyvinylidene fluoride (e.g., KYNAR); ethylenetetrafluoroethylene (e.g., TEFZEL); polytetrafluoroethylene (e.g.,TEFLON, FLUON, DYNEON and NEOFLON). Suitable polyimide films in the tapewrap 304 include KAPTON and ULTEM. Polyetherimides such as SILTEM mayalso be used in the tape wrap 304 for wrapping the cable sensors.

The outer jacket 306 preferably comprises a polymer that can withstandthe temperatures that the optical cable 300 will be subjected to withoutdegrading or reacting with other components in the optical cable 300.The outer jacket 306 is flexible (i.e., it can be mechanically deformedeasily under average human force) and preferably has an elastic modulusof less than 10⁶ pascals when measured at room temperature. The polymerused in the outer jacket 306 is preferably a fluoropolymer, polyimide ora polyetherimide that has a melting point above 150° C., preferablygreater than 200° C. The fluoropolymers used in the outer jacket 306 canbe one of those listed above for the tape wrap 304.

In one embodiment, in one method of manufacturing the optical cable 300,the cable sensors 100A, 100B, 100C and 100D, and so on, are helicallywound in position on the support 302. The winding is maintained at apitch p length that minimizes strain on the optical fiber 102. A tapewrap 304 is then wrapped around the cable sensors 100A, 100B, 100C and100D, and so on. Following the wrapping of the tape wrap 304, the outerjacket 306 may be extruded onto the tape wrap 304. Crosshead extrusionmay be used in the extrusion process to dispose the outer jacket 306onto the tape wrap 304.

The design detailed above has a number of advantages. The materialsselected allow for an operating temperature range of −45° C. to +125°C., which is greater than the operating temperature range of −40° C. to+85° C. for commercially available telecommunications cable. The loosetubed optical fibers are helically wound to create a mechanism thatreduces the stress level that the optical fiber experiences when theoptical cable 300 is stretched or bent. Due to the harsh nature of theoil and gas industry, the sensor will typically be in dynamic motion andexperience levels of strain much greater than that expected for atelecommunications cable. For this design, the strain level is +/−2%.The optical cable 300 disclosed herein may have a length that exceeds 10kilometers, preferably exceeds 20 kilometers, and more preferablyexceeds 50 kilometers.

A unique combination of these optical cable design principles, materialselection and cable manufacturing technique permits the successfulproduction of this small diameter distributed temperature sensing (DTS)optical cable, which operates at a wide temperature range, under highlevels of mechanical stress, while providing accurate temperaturemeasurement in application. This has been achieved without concern forthe surrounding dynamic environmental and mechanical stresses which has,in the past, prohibited the use of optical fiber temperature sensors inthis application.

The optical cable disclosed herein is exemplified by the followingnon-limiting example.

EXAMPLE

This example details an optical cable designed to reduce the mechanicalstrain that the optical sensing fibers experience during installationprocess as a sensor for distributed temperature sensing (DTS) usingBrillouin Optical Time Domain Analyzer/Reflectometry (BOTDA/R).

The optical cable contained 4 optical sensing fibers (cable sensors).The optical cable length is 1 kilometer (km). The pitch diameter D ofthe cable was 2.2 millimeter (mm). The OD of the loose tubes was 1.5 mm.The ID of the loose tubes was 1.0 mm. This ID of 1.0 mm with an opticalsensing fiber outer diameter of 0.250 mm allowed for a value of 0.75 mmfor “d”, the amount of allowable movement of the optical sensing fiberwithin the loose tube. These loose tubes (outside tube), containing theoptical sensing fiber, one fiber per tube, was stranded (wrapped) aroundthe central strength member with a pitch length p of 28.5 mm. Thisallowed for 0% optical fiber strain with up to 2% overall cableelongation (+) or contraction (−). The amount of tensile load requiredto elongate the cable 2% was 356 Newtons (80 pounds force (lbf)).

A 15 meter section of the sensing cable starting at 50 meters in thechart is the section under test. FIG. 5 depicts a graph for the test,where the x-axis is the sensing cable length and y-axis is the Brillouinfrequency. The Brillouin frequency is observed to increase with theelongation strain applied to the section of the sensing cable under testusing a mechanical device. The strain level is calculated based on thechanges in the measured Brillouin frequency. From the chart, it is seenthat the fiber experiences strain with the external applied elongationstrain to the sensing cable. However, its magnitude is significantlyreduced as the outcome of the cable design to ensure glass opticalfiber's mechanical reliability in a high strain environment. It is alsouseful to note the strain level for the section of the cable under testreturns to zero as the applied strain is relieved. This ensures morereliable temperature measurement while the sensing cable is in bothstrain and strain free conditions.

The results shown in the FIG. 5 proves that the design concept inreducing detrimental stress levels to avoid mechanical failure as wellas to improve temperature measurement using BOTDA/R.

While the disclosure has been described with reference to exemplaryembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the disclosure. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the disclosure without departing fromthe essential scope thereof. Therefore, it is intended that theinvention not be limited to the particular embodiment disclosed as thebest mode contemplated for carrying out this disclosure.

What is claimed is:
 1. An optical cable comprising: a support; flexible protective tubes helically wound around the support, each flexible protective tube comprising: an optical fiber comprising: an optical core; a cladding disposed on the core; and a primary coating external to the cladding; and a deformable material surrounding the optical fiber; an outer jacket surrounding the flexible protective tubes; wherein each optical fiber is about 0.5% to about 1.5% longer than its respective flexible protective tube; wherein an allowable strain on the optical cable with substantially zero stress on the optical fibers is determined by equations (1) and (2) below: $\begin{matrix} {{ɛ = {{\underset{\_}{\sqrt{{\pi^{2}\left( {D + \frac{d}{2}} \right)}^{2} + p^{2}}}\mspace{14mu}\underset{\_}{\sqrt{{\pi^{2}\left( {D - \frac{d}{2}} \right)}^{2} + p^{2}}}\mspace{14mu}\underset{\_}{\pi^{2}{dD}}} - \underset{\_}{10\;{dD}}}};} & (1) \\ {{{ɛ \times 100} = {{Percent}\mspace{14mu}{elongation}\mspace{14mu}{or}\mspace{14mu}{contraction}}};} & (2) \end{matrix}$ where d is the amount of optical fiber clearance for free movement within the flexible protective tube, D is an average helical diameter of the helically wound flexible protective tubes, and p is an average helical pitch of the helically wound flexible protective tubes.
 2. The optical cable of claim 1, further comprising a tape wrap that is disposed on the flexible protective tubes and surrounds the flexible protective tubes.
 3. The optical cable of claim 1, where the average helical pitch and the average helical diameter are selected to retain the optical fibers in a state of substantially zero strain.
 4. The optical cable of claim 1, where the average pitch length and the average pitch diameter are selected to retain the optical fibers in a state of ±2% strain.
 5. The optical cable of claim 4, where the support comprises a ductile metal or a polymer.
 6. The optical cable of claim 2, where the tape wrap comprises a polymer that can withstand temperatures of greater than 150° C.
 7. The optical cable of claim 2, where the tape wrap comprises a fluoropolymer, a perfluoropolymer, or copolymers thereof.
 8. The optical cable of claim 1, where the outer jacket comprises a fluoropolymer, a perfluoropolymer, or copolymers thereof.
 9. The optical cable of claim 1, where the deformable material comprises air.
 10. A method of manufacturing an optical cable comprising: helically winding flexible protective tubes around a support, each flexible protective tube comprising: an optical fiber comprising: an optical core; a cladding disposed on the core; and a primary coating external to the cladding; and a deformable material surrounding the optical fiber; and disposing an outer jacket on the flexible protective tubes such that the outer jacket surrounds the flexible protective tubes; wherein each optical fiber is about 0.5% to about 1.5% longer than its respective flexible protective tube; wherein an allowable strain on the optical cable with substantially zero stress on the optical fibers is determined by equations (1) and (2) below: $\begin{matrix} {{ɛ = {{\underset{\_}{\sqrt{{\pi^{2}\left( {D + \frac{d}{2}} \right)}^{*} + p^{2}}}\mspace{14mu}\underset{\_}{\sqrt{{\pi^{2}\left( {D - \frac{d}{2}} \right)}^{*} + p^{2}}}\mspace{14mu}\underset{\_}{\pi^{2}{dD}}} - \underset{\_}{10\;{dD}}}};} & (1) \\ {{{ɛ \times 100} = {{Percent}\mspace{14mu}{elongation}\mspace{14mu}{or}\mspace{14mu}{contraction}}};} & (2) \end{matrix}$ where d is the amount of optical fiber clearance for free movement within the flexible protective tube, D is an average helical diameter of the helically wound flexible protective tubes, and p is an average helical pitch of the helically wound flexible protective tubes.
 11. The method of claim 10, further comprising disposing a tape wrap on the flexible protective tubes such that the tape wrap resides between the outer jacket and the flexible protective tubes.
 12. The method of claim 10, wherein disposing the outer jacket is accomplished via extrusion.
 13. An optical cable comprising: a support; flexible protective tubes helically wound around the support, each flexible protective tube comprising: an optical fiber comprising: an optical core; a cladding disposed on the core; and a primary coating external to the cladding; and a deformable material surrounding the optical fiber; an outer jacket surrounding the flexible protective tubes; wherein each optical fiber is longer than its respective flexible protective tube; wherein an allowable strain on the optical cable produces substantially zero stress on the optical fibers. 